1. Field of the Invention
This invention relates to a method for thermal drying of a solid product in small pieces, whereby a layer of the product in small pieces is brought between a first gas permeable conveyor and a second gas permeable conveyor which both are in contact with the layer which is heated by supplying hot air.
2. Related Art
The invention particularly relates to a method for drying food products such as meals, vegetables, meat or fish, pharmaceutical products, minerals and so on, as far as they are in small pieces and preferably are somewhat deformable
These products can be cut into pieces beforehand, but they can also naturally have the shape of small pieces, such as, for example, peas.
In known methods for thermal drying, the so-called conveyor drying, a stream of rather hot air is supplied over the products on a conveyor. These known methods, however, are relatively slow, whereas the applied high temperatures exert a disadvantageous influence upon the quality of the products.
A method of this kind is disclosed in DE-C-508.308. According to this method, plant materials are fed in a relatively thick layer between two endless perforated conveyors and heated by means of hot air. Hot air may be sucked from one side to the other along the layer. No pressure is exerted by the conveyors.
Another method of this kind, but for drying a filter cake, is disclosed in EP-A-0.651.218. A suspension is fed to an endless perforated conveyor. After filtering, the filter cake is dried by contact with heating elements inside a chamber heated by a gas. In one form the of embodiment, a pressure is exerted on the filter cake by perforated boxes provided with a supplemental heating element inside, which boxes do not move with the conveyor. In other embodiments the filter cake is brought between two perforated conveyors, but then no pressure is exerted. The temperature of the heating gas is as high as possible.
Although thermal drying always is preferred in respect to freeze-drying, as it is cheaper, it, anyhow, requires a lot of energy and has a low thermal efficiency.